This invention relates to vibratory members such as beams or strings and more particularly to isolator means for isolating the vibrations of the vibratory member from its mounts to minimize coupling between the member and its mounts over the range of its frequencies of vibration.
In certain apparatus such as vibrating beam or string accelerometers or pressure transducers, a vibratory member is supported in such a way that forces of acceleration, deceleration or pressure are applied at the ends of the apparatus to change its axial stresses. In an axially unstressed condition, a beam has a certain natural frequency of vibration, determined primarily by its dimensions, the material of which it is constituted, temperature, and the media in which it is operating. In response to an axial stress applied to the beam, the natural frequency of vibration of the beam changes--the frequency increasing in response to axial tension and decreasing in response to axial compression. Similarly, in the case of a vibratory string apparatus, the string is prestressed a predetermined amount greater than the forces of compression it is intended to measure. In such a condition, the beam or string has a certain natural resonant frequency. Axial forces of either compression or tension applied to the ends of the apparatus produce changes in this natural frequency of vibration indicative of the forces so applied.
It is, of course, desirable that the vibration frequency of the vibratory member of the transducer be a true and accurate representation of the axial stress applied to it so that accurate measurements can be obtained. However, in prior art force transducers of this type, this is not the case due to the necessary condition that the vibratory member must be mounted. Mounting the vibratory member, however, permits energy to be lost from the vibratory member to the mounts, thereby making the force transducer as a whole less efficient. This loss of energy results in a decrease in the Q factor of the transducer, that is, the ratio of energy stored in the vibratory system of the force transducer to the ratio of energy lost.
It is desirable to maintain a high Q factor for several reasons. First, in order to operate the force transducer, the vibratory member must be caused to vibrate. Typically, this requires electrical energy. If too much vibratory energy is lost by the vibratory system during operation, relatively high levels of electrical energy must be used in order to maintain a constant level of operation. Such high levels of electrical energy can, however, have detrimental effects to the transducer itself or to other devices located nearby. Additionally, if high levels of electrical energy are required, variations or errors in the electrical power signal will cause larger frequency deviations of the vibratory member thereby resulting in reduced accuracy of the transducer. Second, in the event that the source of electrical energy is momentarily and unexpectedly disconnected, the inability of the transducer to maintain vibratory energy for the time the power is disconnected will cause the vibrations of the vibratory member to dampen quickly, so that when electrical power is restored, inaccurate frequency readings will result.
In a single vibratory member resonator, the vibratory member is directly coupled to the mount. As a result, the frequency of the resonant member is affected by any structural resonances of the member to which the member is mounted. This effect can further seriously degrade the operation of the member. It can limit its operational frequency range and degrade its frequency stability. Moreover, the resonant member becomes sensitive to external vibrations imposed on the housing at any of the housing resonant frequencies; and its temperature coefficient of frequency is affected by the housing temperature coefficient.
One proposal which has been offered to overcome the disadvantages noted above has been the double vibrating member structure wherein the members theoretically vibrate in a push-pull type of action. An attempt is made to construct two members, either beams or strings, identical in size, and the members are mounted parallel to each other. Ideally, the vibrations are such that the members move either simultaneously toward or away from each other whereby end effects are cancelled. However, as a practical matter, the double resonator as an axial stress measuring unit has operating difficulties when an external axial load is applied. If the two members are not loaded equally, the frequency shift due to the externally applied load will be unequal. Under these conditions, there will not be a single well-defined resonant frequency but rather two, one for each member. The existence of two resonant peaks can change the frequency versus load characteristic and can sometimes result in the electronic drive circuitry gain of the vibrating members falling to less than one. This results in the failure of the oscillator loop circuitry to maintain vibrations of the members. Moreover, the beneficial cancelling effects of the double member configuration are dependent on closely matching the dimensions of the two members. If matching is not very close, the cancellation benefits decrease.
Accordingly, under ideal circumstances, the vibratory member's vibration and its changes in vibration should be totally uninfluenced by its mounts so that its changes in vibration would be directly related to the axial stresses applied to it. In such a case, an accurate, reliable accelerometer or other instrument free of interference from its supports could be implemented.
In U.S. Pat. No. 3,470,400 entitled Single Beam Force Transducer With Integral Mounting Isolation which is assigned to the present assignee and is incorporated herein by reference, a force transducer is disclosed in which two isolator masses are used to minimize coupling between the vibratory member of the transducer and the transducer mounts. The isolator masses are connected to the mounts by pairs of spring members. In order to increase the efficiency of the transducer and thereby increase the Q factor, the isolator masses are constructed so that their centers of gravity lie on an axis coincident with the axis of the vibratory member. As a result, the vibrations of the vibratory member are effectively cancelled by the vibrations of the isolator masses so that only a small portion of the vibratory energy of the vibratory member is transmitted to the mounts.
Although this device of the '400 patent is operational and provides satisfactory results, the Q factor of the transducer is not sufficiently high for certain applications. This is due, it is believed, to the fact that too much energy is lost by the vibratory member to the transducer mounts because the isolator masses are not permitted to vibrate freely enough. Since the supports to which the isolator masses are mounted are connected to the end mounts in two places by the two springs, the two springs restrict rotation of the isolator masses. Specifically, referring to FIG. 3 of the '400 patent, the double spring configuration prevents the isolator masses from rotating between the positions shown in broken lines. Although, in theory, the isolator masses would not need to rotate significantly since their shape and position are chosen to eliminate all such vibrations, in reality, such accurate tuning is difficult to attain because of the precise geometrial conditions that must be met. Additionally, having an isolator mass geometry that satisfies the conditions necessary for perfect tuning usually results in other problems such as low isolator stiffness, spurious resonances or both. As a result of such mistuning, the isolator masses will tend to rotate. The double spring arrangement will restrict such rotation thereby allowing too much vibrational energy to be transferred from the vibratory member to the mounts of the transducer and thereby reducing the Q factor.